The VCS to VCVA Converter Appliance is the winning idea from the 2013 Fling Contest. It allows customers to migrate from Windows vCenter Server with an External Microsoft SQL Server Database to the vCenter Server Appliance with an embedded vPostgres database. The Fling migrates the vCenter database, roles, permissions, privileges, certificates and inventory service. The target appliance will run at the same IP address as the source vCenter.
Open source licenseVMware-Migration_fling-0.9-ODP.tar.gz
Also see:

VMware Single Sign On User/Groups are currently not migrated (require re-registration)

Migration Appliance must be able to communicate with the Windows vCenter Server Database and its database as well as the new vCenter Server Appliance. The following ports are used for this communication and should be open on the vCenter Windows server and on the VCSA:
Ports: 22 (ssh), 443 (https), 445 (SMB)

Limitations:

Microsoft SQL Server and vCenter Server must be on separate hosts

Microsoft SQL Express Database is not supported in version 0.9

VMware Single Sign On Users and Groups are not migrated in version 0.9

Windows Local Users and Groups are not migrated in version 0.9

vCenter Alarm action scripts are not migrated in version 0.9

The migration will require some downtime for the vCenter Server

Linked Mode configuration is not migrated. Multiple vCenters must be migrated separately

Any VMware or 3rd party vSphere Web Client plug-ins (e.g. VUM, NSX) that are running on the same host as the vCenter Server will not be migrated